Preparation of superconducting films and a process for preparing the films

ABSTRACT

A novel ceramic substrate useful for the preparation of superconducting films, said substrate having the formula REBa 2  MO 6  where RE represents rare earth metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M represents metals Nb, Sb, Sn, Hf, Zr; and a process for the preparation of superconducting YBa 2  Cu 3  O 7- δ  thick films on new ceramic substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/924,662, filedSep. 5, 1997, now U.S. Pat. No. 5,856,276, which is a continuation ofapplication Ser. No. 08/506,885, filed Aug. 14, 1995, abandoned, whichis a continuation of application Ser. No. 08/221,154, filed Mar. 31,1994, abandoned.

FIELD OF THE INVENTION

This invention relates to novel ceramic substrates for YBa₂ Cu₃ O₇₋δfilms; a process for the preparation of such substrates and a processfor the preparation of superconducting YBa₂ Cu₃ O₇₋δ thick films on thenew ceramic substrates.

BACKGROUND OF THE INVENTION

In the preparation of YBa₂ Cu₃ O₇₋δ (YBCO) thick and thin films, choiceof substrate is a key factor. Chemical non-reactivity between thesubstrate and YBCO is the most crucial for obtaining a superconductinghigh T_(c)(o) film. Besides, for microwave applications, the substrateshould have very low dielectric constant (=10) and the loss factor (10⁻⁴-10⁻⁵) at GHz frequencies [Brandle and Fratello, J. Mater. Res., 5, 2160(1990)]. For epitaxial growth of the film, lattice matching is alsoimportant. MgO is the most widely reported substrate material whichsatisfies the above substrate characteristics reasonably well for YBCOfilms. However, MgO do form an interlayer of Ba salt at the YBCO--MgOinterface if the temperature of processing is above 700° C. [Koinuma etal., Jpn J. Appl. Phys. 27, L1216 (1988), Cheung and Ruckenstein, J.Mater. Res. 4, 1 (1989) and Preng et al., Supercond. Sci. Technol. 3,233 (1990)]. At higher temperatures (>700° C.) chemical interdiffusionof the cation across the film boundary in YBCO--MgO is quite prominent[Hu et al., "Advances in Superconductivity", (Proc. 1st Inter. Symp. onSupercond. ISS 88. Aug. 28-31, 1988, Nagoya, Japan), Edited by K.Kitazawa and I. Ishiguro, Springer-Verlag, Tokyo, p. 647 (1989) and Liet al., J. Mater. Sci., 26, 1057 (1991)]. In particular, in the case ofthick film preparation of YBCO using MgO substrate, where the processingtemperature required is well above 900° C., the Ba salt formation andcation interdiffusion reduce the superconducting transition τ_(c)(o) ofYBCO thick film very appreciably [Bansal, Mater. Lett., 13, 7 (1992)].Other commercially available substrates such as Si, SiO₂, Al₂ O₃,SrTiO₃, GaAlO₃, LaAlO₃ etc. either chemically react with YBCO or have ahigh dielectric constant and loss factor, which makes them unsuitable orless attractive as substrates for YBCO at microwave frequencies. Themain object of the present invention is to produce a process for thepreparation of ceramic substrates of REBa₂ MO₆ (where RE=Rare Earthmetals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M=Nb, Sb,Sn, Hf, Zr) which are chemically compatible with YBCO and have very lowdielectric constant and loss factor making them suitable for microwaveapplications.

SUMMARY OF THE INVENTION

We have now invented novel ceramic substrates useful for the preparationof superconducting YBa₂ Cu₃ O₇₋δ films. These substrates are having aformula REBa₂ MO₆ wherein RE represents metals--Ce, Pr, Nd, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu and M represents metals Nb, Sb, Sn, Hf, Zr.

Another aspect of the present invention relates to a process for thepreparation of new ceramic substrates of the formula REBa₂ MO₆ where RErepresents (rare-earth metals Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu) and M represents metals Nb, Sb, Sn, Hf, Zr, useful for thepreparation of superconducting films.

In addition, we have successfully screen-printed a thick film or YBCOwith a zero transition temperature T_(c)(o) =92 K on these substratesfor the first time.

Thus, the present invention provides novel ceramic substrates, a novelprocess for preparing these substrates and a process for the preparationof superconducting YBa₂ Cu₃ O₇₋δ thick films on these new ceramicsubstrates.

Accordingly, one embodiment of the present invention relates to aprocess for the preparation of new ceramic substrates of the formulaREBa₂ MO₆ where RE represents rare earth metals and M represents metalsNb, Sb, Sn, Hf, Zr useful for the preparation of superconducting filmswhich comprises,

(i) reacting, salts of the rare earth metals, barium and Nb, Sb, Sn, Hf,Zr in an organic medium,

(ii) pressing the resultant mixture in the form of pellets,

(iii) calcining the pellets by heating at a temperature in the range of10000 to 1200° C.,

(iv) repeating the calcination process for 30 to 45 h, preferably 12 hfor each calcination, at temperature in the range of 10000 to 1200° C.till a highly homogeneous mixture is formed,

(v) grinding the calcined material and pelletizing at a pressure in therange of 3 to 6 tons/cm², and

(vi) sintering the resultant product at a temperature in the range of12000 to 1600° C. for a period of 10 to 30 h preferably 20 h and thenfurnace cooled to room temperature.

The salts of the rare earths, barium and other metals used may beselected from oxides, carbonates or nitrates. The purity of the saltsmay be of 99%. The organic medium used may be selected from organicsolvents such as acetone, ethyl alcohol, isopropyl alcohol. Threemultiple calcinations of the pellets may be conducted at temperature10000 to 1200° C. for a period ranging from 10 to 15 h preferably 12 hfor each calcination. The sintering of the final product may effectedfor a period of 10 to 30 h, preferably for 20 h.

In view of the suitability of REBa₂ MO₆ substrates, we have successfullyscreen-printed a thick film of YBCO with a zero superconductingtransition up to 92 K on these substrates.

Thus, yet another aspect of the present invention relates to a processfor the preparation of superconducting YBa₂ Cu₃ O₇ -δ thick films on newceramic substrates of the formula REBa₂ MO₆ where RE represents rareearth metals and M represents metals Nb, Sb, Sn, Hf, Zr, useful for thepreparation of superconducting films which comprises,

(i) mechanically polishing the ceramic substrate of the above saidformula to get highly smooth and shining surfaces,

(ii) preparing thick film of YBCO by known methods,

(iii) screen printing YBCO on said polished REBa₂ MO₆ substrates using amesh size in the range of 325,

(iv) drying the resulting films at a temperature in the range of 1000 to150° C.,

(v) heating the dried films at a rate of 1500 to 250° C./h up to 960° to1000° C. and soaking at this temperature for 1 to 5 minutes,

(vi) cooling the film at a rate of 400° to 600° C./h to bring down thetemperature to 800° C. and keeping the films at this temperature for aperiod of 20 to 40 minutes, and

(vii) cooling the film at a rate of 150° to 250° C./h up to 600° to 400°C. and keeping the film at this temperature for a period of 1 to 2 h.Finally the film is furnace cooled to room temperature.

All the above steps are carried out in the presence of air or flowingoxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing X-ray diffraction patterns of sintered (A)GdBa₂ NbO₆, (B) NdBa₂ SbO₆ and (C) SmBa₂ NbO₆ ;

FIG. 2 is a graph showing X-ray diffraction patterns of (A) YBa₂ Cu₃ O₇₋(B) YBa₂ Cu₃ O₇₋δ --GdBa₂ NbO₆, (C) YBa₂ Cu₃ O₇₋δ- --NdBa₂ SbO₆ and (D)YBa₂ Cu₃ O₇₋δ- --SmBa₂ NbO₆ ;

FIG. 3 is a graph showing X-ray diffraction patterns of screen-printedYBa₂ Cu₃ O₇₋δ thick film on NdBa₂ SbO₆ (substrate peaks are marked by"•");

FIG. 4 is a graph showing X-ray diffraction patterns of screen-printedYBa₂ Cu₃ O₇₋δ thick film on SmBa₂ NbO₆ (substrate peaks are marked by"•");

FIG. 5 is a graph showing the temperature-resistance curve ofscreen-printed YBa₂ Cu₃ O₇₋δ thick film on NdBa₂ SbO₆ substrate; and

FIG. 6 is a graph showing the temperature-resistance curve ofscreen-printed YBa₂ Cu₃ O₇₋δ thick film on SmBa₂ NbO₆ substrate.

DETAILED DESCRIPTION OF THE INVENTION

The details of the invention are described in the Examples given belowwhich are provided by way of illustration only and should not beconstrued to limit the scope of the invention.

EXAMPLE 1 Preparation of Ceramic Substrate of the Formula GdBa₂ NbO₆

GdBa₂ NbO₆ was prepared by solid state reaction method. Gd₂ O₃, BaCO₃and Nb₂ O₅ (purity 99%) were taken in stoichiometric ratio, mixedthoroughly in acetone medium and calcined in air at 1150° C. for 12 hwith two intermediate grindings. The resultant mixture was powdered andpressed at a pressure of 4 tons/cm² in the form of circular pellets andsintered in air at 1450° C. for 20 h.

EXAMPLE 2 Preparation of Ceramic Substrate of the Formula NdBa₂ SbO₆

NdBa₂ SbO₆ was prepared by solid state reaction method. Nd₂ O₃, BaCO₃and Sb₂ O₃ (purity 99%) were taken in stoichiometric ratio, mixedthoroughly in acetone medium and calcined in air at 1100° C. for 10 hwith two intermediate grindings. The resultant mixture was powdered andpressed at a pressure of 5 tons/cm², in the form of circular pellets andsintered in air at 1400° C. for 16 h.

EXAMPLE 3 Preparation of Ceramic Substrate of the Formula SmBa₂ NbO₆

SmBa₂ NbO₆ was prepared by solid state reaction method. Sm₂ O₃, BaCO₃and Nb₂ O₅ were taken in stoichiometric ratio, wet mixed in acetonemedium and calcined in air at 1200° C. for 15 h with two intermediategrindings. Calcined powder was finally, ground and pelletized at apressure of 5 tons/cm² in the form of circular pellets and sintered at1350° C. for 15 h in air.

Structure Determination

The structure of the sintered materials was examined by X-raydiffraction (XRD) method and it is found that all these materials areisostructural having a cubic perovskite structure as shown in the XRDpatterns in FIG. 1 (Sheet No. 1) of the drawings accompanying thisspecification for three typical samples GdBa₂ NbO₆ (Example 1), NdBa₂SbO₆ (Example 2) and SmBa₂ NbO₆ (Example 3). In tables 1a to c we havegiven the computerized XRD data for these materials.

                  TABLE 1(a)                                                      ______________________________________                                        Computerized XRD data GdBa.sub.2 NbO.sub.6                                    No.    2          Intensity                                                                              Width   d    I/Io                                  ______________________________________                                        1      29.780     7988     0.459   2.998                                                                              100                                   2      36.730      594     0.480   2.445                                                                               7                                    3      42.660     2358     0.525   2.118                                                                              30                                    4      52.890     2882     0.555   1.730                                                                              36                                    ______________________________________                                    

                  TABLE 1(b)                                                      ______________________________________                                        Computerized XRD data NdBa.sub.2 SbO.sub.6                                    No.    2          Intensity                                                                              Width   d    I/Io                                  ______________________________________                                        1      29.740     9506     0.375   3.002                                                                              100                                   2      36.690      250     0.210   2.447                                                                               3                                    3      42.510     2776     0.420   2.125                                                                              29                                    4      52.680     3004     0.555   1.736                                                                              32                                    ______________________________________                                    

                  TABLE 1(c)                                                      ______________________________________                                        Computerized XRD data SmBa.sub.2 NbO.sub.6                                    No.    2          Intensity                                                                              Width   d    I/Io                                  ______________________________________                                        1      29.730     7988     0.420   3.003                                                                              100                                   2      36.630      476     0.270   2.451                                                                               6                                    3      42.570     2569     0.435   2.122                                                                              32                                    4      52.780     2750     0.495   1.733                                                                              34                                    ______________________________________                                    

Dielectric Properties

The dielectric properties of the substrate materials were measured at 30Hz to 13 MHz frequencies and values of the dielectric constant (ε') andloss factor (tan δ) at 13 MHz frequency are 24 and 10⁻⁴ respectively atroom temperature. At GHz frequencies the values of ε' and tan δ arefound to be -6 and 10⁻⁵ respectively, which are ideally suitable formicrowave applications.

Chemical Compatibility with YBCO

The most important characteristics of REBa₂ MO₆, which makes thesematerials ideal as substrates for YBCO, is their chemical non-reactivitywith YBCO even at extreme processing conditions. The chemical reactivitybetween YBCO and REBa₂ MO₆ was studied by mixing 1:1 molar mixture ofYBCO and REBa₂ MO₆ and heating the pressed samples at 950° C. for 15 h.In FIG. 2 (sheet No. 2), the XRD pattern of a pure YBa₂ Cu₃ O₇₋δsuperconductor (FIG. 2a) and three representative XRD patterns ofannealed YBa₂ Cu₃ O₇₋δ --GdBa₂ NbO₆ (FIG. 2b). YBa₂ Cu₃ O₇ --NdBa₂ SbO₆(FIG. 2c) and YBa₂ Cu₃ O₇₋δ --SmBa₂ NbO₆ (FIG. 2d) samples mixed in 1:1molar ratio and heated at 950° C. for 15 h have been given. The XRDpatterns clearly show that there is no additional phase formed besidesYBCO and REBa₂ MO₆ in the composite. This indicates that there is noreaction taking place between YBCO and REBa₂ MO₆ even under severe heattreatment. REBa₂ MO₆ are insulating oxides with room temperatureresistivities of the order of 10¹⁰ Ωcm. Detailed percolation studiescarried out on YBCO--REBa₂ MO₆ composites also showed that the presenceof REBa₂ MO₆ up to 60 vol % in the composite did not show anydetrimental effect on the super-conducting transition temperature ofYBCO.

Because of the excellent dielectric properties and chemicalcompatibility of the novel substrates with YBCO as described above, wealso invented a process for the preparation of superconducting YBa₂ Cu₃O₇₋δ thick films on the newly developed ceramic substrates of REBa₂ MO₆as described above. In other words, in view of the suitability of REBa₂MO₆ substrates, we have successfully screen printed a thick film of YBCOwith a zero superconducting transition up to 92 K on these substrates.

We quote a few prior art references regarding the formation ofsuperconducting films here below:

Superconducting thick films have wide applications in microwaveintegrated circuits transmission lines and other microelectronic devices[Alford et al., Supercond. Sci. Technol., 4, 433 (1991), Khare et al.,Supercond. Sci. Technol, 4, 107 (1991) and Alford et al., Nature, 349,680 (1991)]. Screen printing is a straight forward and highly economicalprocess for the deposition of thick films of YBa₂ Cu₃ O₇₋δ (YBCO)superconductors [Budhani et al., Appl. Phys. Lett., 51, 1277 (1987),Shih and Riu, Appl. Phys. Lett., 52, 748 (1988), Koinuma et al., Jpn. J.Appl. Phys., 26, L399 (1987), Bansal and Simon, Appl. Phys. Lett., 53,603 (1988) and Bansal, Mater. Lett., 13, 7 (1992)]. In the preparationof superconducting thick films, the choice of substrate is a key factor.Chemical non-reactivity between the substrates and YBCO is most crucialfor obtaining superconducting films with high transition temperature(≈90 K).

Our process for fabrication of YBCO screen-printed thick film on REBa₂MO₆ substrates is given below:

Before screen-printing YBCO, REBa₂ MO₆ substrates were polishedmechanically to get highly smooth and shining surfaces. Thick film pasteof YBCO, which we used for screen printing was made by mixing YBCOpowder with an organic vehicle and well mixed paste was achieved by handmilling. Film thickness was controlled via viscosity control of YBCOpaste. Thick film of this YBCO paste was then screen-printing on REBa₂MO₆ substrate using a mesh size of 325. Printed films were dried at 100°to 150° C. for 2 to 3 h. Dried films were heated in a programmablefurnace at a rate of 150° to 250° C./h up to 950-1000° C. and soaked atthis temperature for 1-5 min. The films were cooled down at a rate of400°-600° C./h and brought down to 800°-900° C. and kept at thistemperature for 20-40 min and films were then cooled down at a rate of150° C. to 250° C./h up to 600°-400° C. and kept at this temperature for1 to 2 h and after that cooled down to room temperature. All the aboveprocessing were done either in air or in flowing oxygen. The structureof the film was examined by X-ray diffraction method and XRD patterns oftwo typical screen-printed YBCO films on NdBa₂ SbO₆ and SmBa₂ NbO₆substrates are shown in FIGS. 3 and 4 (given in sheet Nos. 3 and 4)respectively. The XRD patterns of these YBCO thick films showed thatexcept for the characteristic peaks of NdBa₂ SbO₆ and SmBa₂ NbO₆substrates, all other peaks could be assigned to a phase pureorthorhombic superconducting YBCO structure.

In the following examples illustrate the preparation of superconductingfilms employing the novel substrates:

EXAMPLE 4 Preparation of Superconducting Film Using Ceramic Substrate ofthe Formula NdBa₂ SbO₆

A YBa₂ Cu₃ O₇₋δ paste was made by mixing fine YBa₂ Cu₃ O₇₋δ powder withorganic vehicle and its viscosity was controlled by adding fish oil.This paste was screen-printed on a highly polished NdBa₂ SbO₆ substrateusing a screen of 325 mesh size. The film, after drying at 100° C. for90 minutes was heated in a programmable furnace at a rate of 3°C./minute up to 980° C. and soaked at this temperature for 5 minutes. Itwas then cooled at a rate of 10° C./minute to bring down the temperatureto 900° C. and kept at 900° C. for 30 minutes. It was then cooled to600° C. at a rate of 4° C./minute and again kept at this temperature for60 minutes. The film was finally furnace cooled to room temperature.Entire processing was carried out in air.

EXAMPLE 5 Preparation of Superconducting Film Using Ceramic Substrate ofthe Formula SmBa₂ NbO₆

A YBa₂ Cu₃ O₇₋δ paste was made by mixing fine YBa₂ Cu₃ O₇₋δ powder withorganic vehicle and its viscosity was controlled by adding fish oil. Thepaste was screen-printed on highly polished SmBa₂ NbO₆ substrate using ascreen of 325 mesh size. The film, after drying at 120° C. for 60minutes was heated in a programmable furnace at a rate of 3° C./minuteup to 980° C., soaked at this temperature for 5 minutes. It was thencooled at a rate of 8° C./minute to bring down the temperature to 900°C. for 30 minutes. It was then cooled to 550° C. at a rate of 3°C./minute and again kept at this temperature for 60 minutes. The filmwas finally furnace cooled to room temperature. The entire processingwas done in air.

The structure of the film was examined by X-ray diffraction method. TheXRD patterns of two typical screen-printed YBCO films on NdBa₂ SbO₆(Example 4) and SmBa₂ NbO₆ (Example 5) substrates are shown in FIGS. 3and 4 of the drawings (Sheet Nos. 3 and 4) accompanying thisspecification. The XRD pattern of these YBCO thick films showed thatexcept for the characteristic peaks of NdBa₂ SbO₆ and SmBa₂ NbO₆substrates, all other peaks could be assigned to a phase pureorthorhombic superconducting YBCO structure.

Superconductivity in these YBCO screen-printed thick films on REBa₂ MO₆substrates were studied by temperature-resistance measurements. Thefilms show a metallic behavior in the normal state and givesuperconducting transition temperature of 88-94 K (T_(c) onset) and85-92 K (T_(c)(o) as shown in the two representative examples given inFIGS. 5 and 6 (sheet Nos. 5 and 6). These films give a current densityof the order of 10⁵ A/cm².

What is claimed is:
 1. A process for the preparation of a ceramicsubstrate of the formula REBa₂ MO₆ where RE represents rare earth metalsand M represents metals Nb, Sb, Sn, Hf, and Zr, which processcomprises:(i) reacting salts of rare earth metals, barium, and a metalselected from the group consisting of Nb, Sn, Sb, Hf, and Zr in anorganic medium to form a mixture, (ii) forming pellets of the resultantmixture, (iii) calcining the pellets to form a homogenous mixture; (iv)grinding the calcined material; (vi) sintering the resultant product;and (vii) allowing the resultant product to cool to room temperature toobtain the ceramic substrate.
 2. The process of claim 1 wherein thepellets are calcined by heating at a temperature in the range of 1000°to 1200° C. for a period ranging from 10 to 15 h.
 3. The process ofclaim 1 wherein the calcined material is ground at a pressure of 3 to 6tons/cm².
 4. The process of claim 1 wherein the product is sintered in afurnace at a temperature in the range of 1200° C. to 1600° for a periodof 10 to 30 h.
 5. A process for the preparation of a ceramic substrateof the formula REBa₂ MO₆ where RE represents rare earth metals and Mrepresents metals Nb, Sb, Sn, Hf, and Zr, which process comprises:(i)reacting salts of the rare earth metals, barium, and a metal selectedfrom the group consisting of Nb, Sn, Sb, Hf, and Zr in an organic mediumto form a mixture, (ii) pressing the resultant mixture in the form ofpellets, (iii) calcining the pellets by heating at a temperature in therange of 1000° to 1200° C. for a period ranging from 10 to 15 h, (iv)repeating the calcination process of step (iii) twice till a highhomogeneous mixture is formed, (v) grinding the calcined material andpelletizing at a pressure of 3 to 6 tons/cm², (vi) sintering theresultant product in a furnace at a temperature in the range of 1200° C.to 1600° for a period of 10 to 30 h, and (vii) then allowing theresultant product to cool to room temperature in the furnace.
 6. Theprocess as claimed in claim 5 wherein the salts of the rare earths,barium and other metals used is selected from the group consisting ofoxides, carbonates, and nitrates.
 7. The process as claimed in claim 5wherein the purity of the salts is 99%.
 8. The process as claimed inclaim 5 wherein the organic medium used is acetone or an alcohol.
 9. Theprocess a claimed in claim 5 wherein the multiple calcination of thepellets is effected for a period of 12 h for each calcination.
 10. Theprocess as claimed in claim 5 where the sintering of the final productis effected for a period of 20 h.
 11. The process of claim 1, whichfurther comprises:(i) mechanically polishing the ceramic substrate toprovide a smooth, shining surface thereon, (ii) preparing a film of YBCOby mixing YBCO with an organic vehicle, (iii) printing the YBCO film onthe polished substrate; and (iv) drying the printed film to obtain thesuperconducting YBa₂ Cu₃ O₇₋δ film on a ceramic substrate, wherein allthe above steps are carried out in the presence of air or of flowingoxygen to provide a superconducting YBa₂ Cu₃ O₇₋δ film on the substrate.12. The process as claimed in claim 11 where the printing of the film isconducted by screen printing using a mesh size in the range of
 325. 13.The process as claimed in claim 11 where the film is dried at atemperature in the range of 100° C. to 150° C.
 14. The process asclaimed in claim 11 where the dried film is heated at a rate of 150° C.to 250° C./h up to a maximum temperature of 950° C. to 1000° C. and thenis soaked at this temperature for 1 to 5 minutes.
 15. The process asclaimed in claim 14 which further comprises cooling the heated film at arate of 400° to 600° C./h to reduce the temperature to 800° to 900° C.and maintaining the film at this temperature for a period of 20 to 40minutes.
 16. The process as claimed in claim 14 where the film isfurther cooled the film at a rate of 150° C. to 250° C./h down to 600°to 400° C. and is maintained at this temperature for a period of 1 to 2h.
 17. The process of claim 1, which further comprises:(i) mechanicallypolishing the ceramic substrate of the above said formula to get highsmooth and shining surfaces, (ii) preparing thick film of YBCO by mixingYBCO with an organic vehicle, (iii) screen-printing the thick film ofYBCO on REBa₂ MO₆ substrate using a mesh size in the range of 325, (iv)drying the resulting films at a temperature in the range of 100° C. to150° C., (v) heating the dried films at a rate of 150° C. to 250° C./hup to a maximum temperature of 950° C. to 1000° C. and soaking at thistemperature for 1 to 5 minutes,(vi) cooling the film at a rate of 400°to 600° C./h to bring down the temperature to 800° to 900° C. andkeeping the films at this temperature for a period of 20 to 40 minutes,and (vii) cooling the film at a rate of 150° C. to 250° C./h down to600° to 400° C. and keeping the film at this temperature for a period of1 to 2 h, wherein all the above steps are carried out in the presence ofair or of flowing oxygen to provide a superconducting YBa₂ Cu₃ O₇₋δ filmon the substrate.